1. Field of the Invention
The present invention relates to focus adjustment control for an imaging apparatus.
2. Description of the Related Art
The introduction of single-lens reflex cameras has generated the opportunity to capture moving images with a large imaging sensor (for example, a 35 mm full-size sensor). Since a large sensor exhibits a shallow depth of field in comparison to small sensors that are used in conventional video cameras, blurring of an image may result. Therefore, in addition to in-focus areas of the image, prominent out-of-focus areas in the image are a cause of concern, and therefore there is a demand for further improvement to auto-focus adjustment.
A so-called TV-AF (autofocus) method is known as an auto-focus adjustment method during capture of moving images in which the sharpness of the imaging signal is evaluated. In this method, focus positions having the highest apparent focus are searched and extracted in order to evaluate the sharpness of the imaging signal itself. However, since the absolute defocus amount is not known, time is required to reach the in-focus point. In other words, acquisition of the in-focus point is retarded since the in-focus direction is only known from information related to the focus lens position in that time range and the difference in the level of the TV-AF signal. The focus lens must be constantly moved to compare with the level of the TV-AF signal, and therefore changes in the degree of blurring in the resultant image are emphasized particularly in objects that are removed from the depth of field.
A method has been proposed for detecting a focus state by applying a pupil division to a light flux in pixels in a portion of an imaging sensor and causing the divided light flux to become incident upon a pair of detection pixels (refer to Japanese Patent Laid-Open No. 2000-292686). Currently, progress in improved pixelation and the increasing size of imaging sensors have enabled the incorporation of focus state detection areas into a plurality of areas in the screen. In this manner, since a defocus amount of each area is discernable, the distance to an object to be imaged can be known in relation to each area in the screen. However, since the pixels in a portion of the imaging sensor are used as detection pixels, that is to say, as focus state detection elements, image data in relation to that portion must be generated by an image interpolation process, or the like. Therefore, there is a risk that the captured image will be degraded by an increased density caused by disposing a detection element on the whole effective pixel region in order to increase the focus detection accuracy.
A conventional automatic focus adjustment apparatus cannot realize an automatic focus adjustment capability that is adapted to an imaging system that has a shallow depth of field. As a result, a conspicuous change in an amount of blurring results in a TV-AF method, and when using a configuration in which a focus state detection element is incorporated into the imaging sensor, if the arrangement density of the detection elements is reduced to prevent image quality degradation, a sufficient detection accuracy is not obtained.